In a letter accompanying its directive against the ordination of men with "deep-seated homosexual tendencies," the Vatican has told bishops that gay priests should not teach in Roman Catholic seminaries.

The Rev. Donald B. Cozzens, a Catholic author and former seminary rector, called the letter a "bombshell" because it affects current priests, not just future ones.

Some experts on church law said yesterday that the letter is nonbinding and can simply be ignored by bishops. But others predicted that it will usher in a gradual purge of gays from leadership positions in the church, even if they have kept their vows of celibacy.

Because priests who teach in seminaries are frequently transferred to serve in parishes and vice versa, "it could be implemented gradually, without anybody knowing" for certain why a clergyman was moved, said Sister Katarina Schuth, a professor at University of St. Thomas in Minnesota who is a leading researcher on Catholic seminaries.

"It's an amazing statement coming as it does on the heels of so many people assuring us that this document does not relate to priests already ordained," said the Rev. James Martin, a Jesuit priest in New York.

The two-page cover letter, dated Nov. 4, was sent to bishops around the world by the Congregation on Catholic Education, the Vatican department responsible for seminaries. It accompanied a six-page "instruction" that said men who "practice homosexuality, present deep-seated homosexual tendencies or support the so-called 'gay culture' " should not be admitted to seminaries or ordained as priests.

The instruction was published by the Vatican on Tuesday, but the letter was not. Its contents were first reported by the Catholic News Service on Wednesday, and the full text was obtained by The Washington Post yesterday.

Most of the letter is devoted to the bureaucratic history of the instruction, emphasizing the heavy involvement of the department formerly headed by Pope Benedict XVI, known as the Congregation for the Doctrine of the Faith.

The letter says that the instruction "does not call into question the validity of the ordination" of men with "homosexual tendencies" who are priests. "They are to continue to exercise their ministry, taking care to live with integrity," it says. But it adds: "Because of the particular responsibility of those charged with the formation of future priests, they are not to be appointed as rectors or educators in seminaries."

The letter was signed by the top officials of the education congregation, Polish Cardinal Zenon Grocholewski and Canadian Archbishop J. Michael Miller.

The Rev. Ladislas Orsy, a canon lawyer at Georgetown University, said the cover letter does not have the same legal authority as the instruction.

"What kind of canonical force does it have? I'm speaking professionally, not editorializing. The honest answer is none," he said. "It's not a piece of legislation. It would be like a letter from a committee chairman in Congress."

Orsy said the letter should be viewed as nonbinding advice to bishops that concerns only future appointments of seminary instructors, because "we have a sweeping principle in canon law that no rule is retroactive unless it specifically says so."

Nevertheless, several Catholic scholars said the letter is a logical step from the instruction.

"Priests still struggling with homoerotic desires are perhaps not the best-placed to act as evaluators or counselors of candidates for the priesthood who are working through similar issues," said papal biographer George Weigel.

Cozzens, whose survey research indicates that a quarter to half of all U.S. priests are gay, said the letter "doesn't say that rectors or professors in our seminaries who have already been appointed should be removed, but one wonders if that's not what might begin to happen -- a kind of culling of gay rectors and professors."

Furthermore, he said, "I think it could also raise questions about people working in chanceries and about bishops who happen to be gay. And why stop there? I see it as a logical extension of the instruction, but it underscores the problematic nature of the instruction."